linux-kernel - Re: [PATCH v2 0/2] KVM: fixes for the kernel-hardening tree

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Date:   Fri, 27 Oct 2017 16:25:03 +1100
From:   Paul Mackerras <paulus@...abs.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        James Hogan <james.hogan@...tec.com>,
        kernel-hardening@...ts.openwall.com,
        Kees Cook <keescook@...omium.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v2 0/2] KVM: fixes for the kernel-hardening tree

On Thu, Oct 26, 2017 at 03:45:45PM +0200, Paolo Bonzini wrote:
> Four KVM ioctls (KVM_GET/SET_CPUID2 on x86, KVM_GET/SET_ONE_REG on
> ARM and s390) directly access the kvm_vcpu_arch struct.  Therefore, the
> new usercopy hardening work in linux-next, which forbids copies from and
> to slab objects unless they are from kmalloc or explicitly whitelisted,
> breaks KVM on those architectures.
> 
> The kvm_vcpu_arch struct is embedded in the kvm_vcpu struct and the
> corresponding slab cache is allocated by architecture-independent code.
> It is enough, for simplicity, to whitelist the whole sub-struct and
> only touch one place of the KVM code.  Later, any further restrictions
> can be applied in the KVM tree.

I checked arch/powerpc/kvm, and all the copy_to/from_user calls are
accessing the stack or memory allocated with kzalloc or kvzalloc, so
if I understand correctly, we should be OK there.

Paul.